Due to higher functionality of wireless systems, a method of improving a transmission capacity by using a CDMA signal or orthogonal multiplexing time division such as OFDM has become the mainstream. It is known that, when such a multiplexed signal is amplified, an output signal having power of four or more times average power arises in a time component of the output signal. At this time of amplification, an output characteristic of a non-linear amplifier may be distorted, and sufficient signal amplification and demodulation may not therefore be achieved. Then, an amplifier traditionally referred to as a Doherty-type amplifier based on a Doherty operation, which efficiently operates, has been proposed and put to practical use.
A configuration of the traditional Doherty-type amplifier is shown in FIG. 4. The traditional Doherty-type amplifier is formed of a carrier amplifier 102 that operates in class AB mode in which linear amplification is performed, and a peak amplifier 103 that operates in B or C mode in which power equal to or more than a certain input level is amplified. An input signal is divided by power distribution, and a divided signal is usually shifted with a phase difference (normally 90 degrees) by a phase shifter 101. The signal with the phase difference is supplied to the peak amplifier 103. To an output side of the carrier amplifier 102, a quarter wavelength impedance transformation circuit 104 is connected. After being combined with the signal from the peak amplifier 103, the resulting signal is connected to a load 110 through the quarter wavelength impedance transformation circuit 109.
In such a Doherty amplifier, when the peak amplifier 103 is turned off, the impedance on the side of the peak amplifier 103 seen from a power combining point is substantially infinite. In this case, by setting a characteristic impedance of the quarter wavelength impedance transformation circuit 104 on the output side of the carrier amplifier 102 to double that of the load 110 and setting a characteristic impedance Zc of the quarter wavelength impedance transformation circuit 109 after power combining to Zc=RL/√{square root over (2)}, the impedance on the output side of the carrier amplifier 102 is increased to 2RL.
At this time, only the carrier amplifier 102 operates in a high-impedance state, and power consumption of the peak amplifier 103 is substantially zero. Accordingly, an operation with an efficiency higher than that of a B-class amplifier becomes possible particularly at an output level with a large back-off amount.
Next, when the peak amplifier 103 is turned on by a certain input signal, the impedance at the power combining point assumes RL/2. Accordingly, the load impedance of each of the carrier amplifier 102 and the peak amplifier 103 is transformed to RL by the quarter wavelength impedance transformation circuit (having a characteristic impedance of RL/20.5). It means that an impedance Z2 for the output side of the carrier amplifier 102 varies as follows:                Z2=2RL: when the peak amplifier is turned off        Z2=RL: when the peak amplifier is turned on        
When the peak amplifier is turned on, a high output characteristic is obtained by a power combining operation by the two power amplifiers. When the peak amplifier operates, due to addition of a high efficiency characteristic of the peak amplifier, deterioration of an efficiency characteristic as an entirety of the Doherty amplifier is small.
Examples of an amplifier in which a quarter wavelength line is not used for an impedance transformation circuit are disclosed in Patent Documents 1, 2, and 3.    Patent Document 1:    JP Patent Kokai Publication No. JP-P2006-197556A    Patent Document 2:    JP Patent Kokai Publication No. JP-P2006-332829A    Patent Document 3:    JP Patent Kokai Publication No. JP-P2006-345341A